1. Field of the Invention
The present invention relates to a television receiving apparatus having a receiving circuit that can receive both a digital television signal and an analog television signal.
2. Description of the Related Art
In recent years, digital television broadcasts such as digital CS (Communication Satellite) broadcast, BS (Broadcast Satellite) broadcast, and digital ground wave broadcast have been performed. It is predicted that analog television broadcasts will be gradually shifted to digital television broadcasts.
However, even most television broadcasts have been shifted to digital broadcasts, existing analog television broadcasts cannot be stopped. Thus, in the transient period of which the analog television broadcasts are shifted to the digital television broadcasts, both analog television digital broadcasts and digital television digital broadcasts will coexist.
When both analog television broadcasts and digital television broadcasts are performed, television receivers that can receive both analog television broadcasts and digital television broadcasts are desired.
FIG. 1 is a block diagram showing an example of the structure of a receiving circuit of a conventional television receiver. The receiving circuit can receive both digital television broadcasts and analog television broadcasts.
Referring to FIG. 1, an RF (Radio Frequency) signal is received from an antenna (not shown). The RF signal is supplied to an antenna input terminal 101. The signal that is input from the antenna input terminal 101 is supplied to a power splitter 102. The power splitter 102 splits the received signal into an analog broadcast signal and a digital broadcast signal.
The analog broadcast signal is supplied from the power splitter 102 to an analog broadcast tuner 103.
The analog broadcast tuner 103 selects a signal having a desired carrier frequency from the RF signal received through the power splitter 102. The selected signal is converted into an IF (Intermediate Frequency) signal. A frequency set signal is supplied from a controller 115 to the analog broadcast tuner 103. Corresponding to the frequency set signal, the oscillation frequency of a PLL (Phase Locked Loop) synthesizer of the analog broadcast tuner 103 is set. Corresponding to the oscillation frequency, the reception frequency is set. An AGC control signal is supplied from an analog demodulating circuit 108 to an AGC (Automatic Gain Control) circuit of the analog broadcast tuner 103.
An output of the analog broadcast tuner 103 is supplied to an SAW (Surface Acoustic Wave) filter 107. The SAW filter 107 has a pass band for the IF signal received from the analog broadcast tuner 103. The SAW filter 107 should have an excellent characteristic for the analog broadcast.
An output of the SAW filter 107 is supplied to an analog demodulating circuit 108. The analog demodulating circuit 108 amplitude-demodulates the analog television signal to an NTSC format video signal. The demodulated NTSC format analog video signal is output from an output terminal 109.
The digital broadcast signal is supplied from the power splitter 102 to a digital broadcast tuner 104.
The digital broadcast tuner 104 selects a signal having a desired carrier frequency from the RF signal received through the power splitter 102 and converts the selected signal into a predetermined IF (Intermediate Frequency) signal. A frequency set signal is supplied from the controller 115 to the digital broadcast tuner 104. Corresponding to the frequency set signal, the oscillation frequency of a PLL synthesizer of the digital broadcast tuner 104 is set. Corresponding to the oscillation frequency, the reception frequency is set. An AGC control signal is supplied from a digital demodulating circuit 112 to an AGC circuit of a digital broadcast tuner 106.
An output of the digital broadcast tuner 104 is supplied to an SAW filter 110. An output of the SAW filter 110 is supplied to an SAW filter 111. The SAW filters 110 and 111 have a pass band for the IF signal received from the digital broadcast tuner 104. A filter composed of the SAW filters 110 and 111 accomplishes an optimum characteristic for the digital broadcast.
An output of the SAW filter 111 is supplied to the digital demodulating circuit 112. The digital demodulating circuit 112 performs a demodulating is process such as QAM (Quadrature Amplitude Modulation), VSB (Vestigial Sideband Amplitude Modulation), or OFDM (Orthogonal Frequency Division Multiplex). An output of the digital demodulating circuit 112 is output from an output terminal 113.
Thus, according to the related art reference, the analog broadcast tuner 103 and the digital broadcast tuner 104 are independently disposed. The analog broadcast signal and the digital broadcast signal are split by the power splitter 102.
This is because the required characteristic for the analog broadcast signal is different from that for the digital broadcast signal.
In other words, in the analog broadcast, since a video signal is amplitude-modulated, a high video amplitude characteristic is desired. In addition, a high S/N ratio is required. On the other hand, in the digital broadcast, since QAM, VSB, OFDM, or the like is used, an excellent phase characteristic is required. Moreover, in the digital broadcast, a filter having a sharp characteristic is required. In the analog broadcast, when a filter having a sharp characteristic is used, the resolution deteriorates.
To solve such a problem, the power splitter 102 splits the received signal into an analog broadcast signal and a digital broadcast signal. The split analog broadcast signal is processed by the analog broadcast tuner 103, the SAW filter 107, and the analog demodulating circuit 108. The split digital broadcast signal is processed by the digital broadcast tuner 104, the SAW filters 110 and 111, and the digital demodulating circuit 112.
The SAW filter 107 has an optimum characteristic for the analog broadcast.
The SAW filters 110 and 111 have optimum characteristics for the digital broadcast. In the digital system, the SAW filter 110 and the SAW filter 111 are tandem-connected so that a sharp characteristic for the digital broadcast can be obtained.
In the receiving circuit of the conventional television receiver that receives both a conventional analog broadcast and a digital broadcast, since the power splitter 102 splits the RF signal into an analog broadcast signal and a digital broadcast signal, a power loss takes place in the power splitter and thereby the S/N ratio deteriorates.
In addition, the receiving circuit of the television receiver that receives both a conventional analog broadcast and a digital broadcast is provided with the analog broadcast tuner 103 and the digital broadcast tuner 104. In addition, the receiving circuit is provided with the SAW filter 107 for the analog broadcast and the SAW filters 110 and 111 for the digital broadcast. Thus, the circuit scale becomes large.
Therefore, an object of the present invention is to provide a television receiving apparatus that can receive both an analog television broadcast and a digital television broadcast and that allows the circuit scale to be reduced.
Another object of the present invention is to provide a television receiving apparatus that can receive both an analog television broadcast and a digital television broadcast and that allows the S/N ratio to be improved.
The present invention is a television receiving apparatus for receiving an analog television broadcast and a digital television broadcast, comprising a tuner means for receiving both an analog television broadcast signal and a digital television broadcast signal, selecting a signal having a desired carrier frequency from the received signals, and converting the selected signal having the desired carrier frequency into an intermediate frequency signal, an analog demodulating means for demodulating the analog television broadcast signal to a video signal, a digital demodulating means for demodulating the digital television broadcast signal to a base band signal, a first filter means disposed downstream of the tuner means, a second filter means disposed between the first filter means and the analog demodulating means, and a third filter means disposed between the first filter means and the digital demodulating means.
According to the present invention, an analog/digital common tuner is disposed. The analog/digital common tuner has a flat frequency characteristic in the channel selection band for an analog broadcast. A first SAW filter is disposed downstream of the tuner. A second SAW filter is disposed between the first SAW filter and an analog demodulating circuit. A third SAW filter is disposed between the first SAW filter and a digital demodulating circuit. When an analog broadcast is received, the first SAW filter and the second SAW filter form a filter having a characteristic necessary for the analog broadcast. When a digital broadcast is received, the first SAW filter and the third SAW filter form a filter having a characteristic necessary for the digital broadcast. Since the first SAW filter is shared for both the analog broadcast and the digital broadcast, the circuit scale can be reduced. In addition, since an analog broadcast signal and a digital broadcast signal are not split by a power splitter, the S/N ratio can be improved.
The analog demodulating circuit outputs an AGC control signal for an analog broadcast. The digital demodulating circuit outputs an AGC control signal for a digital broadcast. The switch circuit selectively supplies the AGC control signal for the analog broadcast and the AGC control signal for the digital broadcast to the analog/digital common tuner. Thus, when an analog broadcast is received, the reception gain for the analog broadcast can be quickly and optimally set. When a digital broadcast is received, the reception gain for the digital broadcast can be quickly and optimally set.
Likewise, the analog demodulating circuit outputs an AFT control signal for an analog broadcast. The digital demodulating circuit outputs an AFT control signal for a digital broadcast. The switch circuit selectively supplies the AFT control signal for the analog broadcast and the AFT control signal for the digital broadcast to the controller. Thus, depending on which of an analog broadcast or a digital broadcast is received, the AFT control can be optimally performed.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.